Fusion tags for proteins are used for a variety of purposes, such as improved protein solubility, detection, purification, expression, and localization.
Common fusion tags used for protein detection include green fluorescent protein (GFP) and its many variants (Tsien 1998). However, a problem with GFP and its variants is that they are not particularly suitable for all cellular compartments. In oxidizing environments such as the eukaryotic secretory pathway, the mitochondrial inner membrane space, and the periplasm of gram-negative bacteria, for example, GFP and its variants have a tendency to oligomerize, misfold, cause misfolding of fused target proteins, or cause abnormal trafficking of fused target proteins. Fluorescent fusion tags suitable for oxidizing environments and other cellular compartments are needed.
Common fusion tags for use in increasing expression and solubility of proteins in bacteria include maltose binding protein (MBP), small ubiquitin-like modifier (SUMO), and glutathione S-transferase (GST), among others (Bell et al. 2013, Butt et al. 2005). However, few comparable tools are available for mammalian protein expression. Two commercially available mammalian tags are SUMO and Fc. Unfortunately, SUMO is restricted to N-terminal fusions (Marblestone et al. 2006), which limits its utility, especially for membrane proteins. Fc fusion can improve solubility of proteins, but is not reported to enhance expression levels. Fusion tags that significantly increase protein yield, particularly in mammalian cells, are needed.